A. Technical Field
The present invention relates to a light emitting diode (hereinafter, “LED”) illumination system, and more particularly, to systems, devices and methods of rapidly ramping up a transformer current and a LED driver current by coupling a transformer load compensation circuitry to an output of an electronic transformer.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become as bright and more efficient than other lighting technologies, such as halogen lamps. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths, and suitable for white illumination. Lifetime of LEDs is also extended to more than 10 thousand hours, and can work at input powers up to many watts.
When compared to halogen lamps, LEDs are relatively smaller, and have a longer operating life. Moreover, LEDs also require significantly less amount of power to operate than a typical halogen lamp, e.g., MR16. For example, a halogen lamp may operate within a range of 20-50 Watts, while an LED at about 5-10 Watts is sufficient to provide a similar level of brightness.
FIG. 1 illustrates an exemplary illumination system 100 in which a halogen lamp is used. In this example, a high voltage AC signal 110 is delivered to an electronic transformer 120, which converts the signal to a 12V AC signal 130. This 12V AC signal 130 is provided to drive a halogen lamp 140 to generate light.
The conversion of the AC signal by the electronic transformer 120 from a high voltage signal to a 12V signal causes chopping or clipping on the 12V output signal envelope. The AC supply voltage 110 that directly comes from the wall outlet is normally associated with a sinusoidal signal 150 having amplitude of 110V or 220V at 60 Hz. As shown, the electronic transformer 120 converts the AC supply voltage 110 to a high-frequency signal 130 having an associated signal envelope 160. The electronic transformer 120 comprises internal circuitry that starts at an effective turn-on voltage, which causes clipping 164, 165, 169, 170 on both the leading and trailing edges of the signal envelope. Until the input signal 150 reaches a minimum voltage threshold, the electronic transformer is effectively off, which results in this clipped output waveform.
In the illumination system 110, the halogen lamp may be replaced with a LED or a LED string in which LEDs are connected in series. Each power LED in the LED string requires a nominal current anywhere in the range of 35-1400 mA, a forward voltage drop of 3V and large manufacturing tolerances. For a comparable illumination level, a MR16 halogen lamp of 20 W may be replaced with a single 5 W MR16 LED or a 5 W MR16 LED string. Although power consumption and energy efficiency has been largely improved, the low input power level may cause a failure in operation of the electronic transformer 120.